The present invention relates to a color printer which uses a plurality of Raster Output Scanner (ROS) imagers to form sequential, registered color images on the surface of a photoreceptor belt moving through imaging stations associated with each imager. More particularly, the present invention is related to use of a quadrant photodetector cell which provides the dual functions of locating the start and end of each ROS scan line, as well as detecting and correcting for registration errors in the process direction.
In recent years, digital color printers have been increasingly utilized to produce output copies from input video data representing original image information. The printer may use a plurality of imagers, either ROS or LED print bars, to expose the charged portions of a photoconductive member to record an electrostatic latent image thereon. Generally, a ROS incorporates a laser for generating a collimated beam of monochromatic radiation. The laser beam is modulated in conformance with the image information. The modulated beam is incident on a scanning element, typically a rotating polygon having mirrored facets. The light beam is reflected from each facet and thereafter focused to a spot on the photosensitive member. The rotation of the polygon causes the spot to scan linearly across the photoconductive member in a fast scan (i.e., line scan) direction. Each scan line crosses a start of scan (SOS) sensor and an end of scan (EOS) sensor, which regulates the image forming areas of the exposed image. Meanwhile, the photoconductive member is advanced relatively more slowly in a slow scan direction which is orthogonal to the fast scan direction. In this way, the beam scans the recording medium with a plurality of scan lines in a raster scanning pattern. The time between the formation of each scan line is constant due to the uniformly rotating nature of the polygon. Thus, such operation is characterized as synchronous. The light beam is intensity-modulated in accordance with an input image serial data stream, at a rate such that individual picture elements ("pixels") of the image represented by the data stream are exposed on the photoconductive member to form a latent image, which is then developed and transferred to an appropriate image receiving medium such as paper.
Color digital printers may operate in either a single pass or multiple pass mode. In a single pass, process color system, three or four ROS imagers are positioned adjacent to a moving photoreceptor surface and are selectively energized to create successive image exposures, one for each of the three process colors, cyan, magenta and yellow. A fourth black imager is usually added. A color digital printer may also operate in a highlight color mode wherein one or two colored images and black are exposed.
In a multiple pass, process color system, each image area on the photoreceptor surface must make at least three revolutions (passes) relative to the transverse scan lines formed by the modulated beam generated by the imagers. With either system, each image is typically formed within a tight tolerance of .+-.0.05 mm. Each color image must be registered in both the photoreceptor process direction and in the direction perpendicular to the process direction (referred to as fast scan, lateral or transverse registration).
Techniques for registration in the process direction of ROS systems are known in the prior art and disclosed, for example, in copending application, U.S. Ser. No. 07/946,690 filed on Sep. 18, 1992, U.S. Ser. No. 07/951,744, filed on Sep. 25, 1992 , and U.S. Ser. No. 07/923,925, filed on Aug. 20, 1992, all assigned to the same assignee as the present invention, all of whose contents are hereby incorporated by reference.